Physical characteristics

Squamates are ectotherms, obtaining their bodily heat from the external environment rather than generating it metaboli-cally in the manner of endotherms (birds and mammals). Ec-tothermy is sometimes seen as a disadvantage, and lay people often erroneously refer to snakes and lizards as "cold-blooded." Snakes and lizards regulate their body temperatures behav-iorally. When it is cold, they actively bask and seek out warm microclimates. When it is too hot, they look for shade and cooler places. When they are active, many squamates have body temperatures just as high as those of birds and mammals. In fact, ectothermy has real advantages over endothermy, especially in warm, dry, unproductive environments, such as deserts and semiarid regions. Ectotherms enjoy a low-energy lifestyle that endotherms can only envy. The food consumed by a 0.17-oz (5 g) bird in a single day will last a 0.17-oz squamate a month. An unproductive habitat, where endotherms cannot exist, will support viable populations of ectotherms. Snakes can go for many months, or even an entire year, without feeding, simply by allowing the body temperature to drop. The economic lifestyle offered by ectothermy enables squamates to thrive and persist during droughts and periods of resource shortages, which a high-energy endotherm simply could not tolerate.

Morphologic features

All lizards and snakes shed their skins at least once each year. Before shedding, a new inner layer of skin forms and separates from the outermost layer of older skin, which appears pale. Eye spectacles of snakes become cloudy. Snakes hook their lip scales on an immobile object and crawl out of their skins, leaving them behind intact and inside out. Sometimes, a snake can be identified to a species from its shed skin. Most lizards shed their skins in patches.

Most lizards have fracture planes in the caudal vertebrae, which facilitate tail loss. Some can even autotomize their own tails voluntarily. Tail loss often allows a lizard to escape from a would-be predator. In such species, tails are regenerated rapidly, although the regrown tail is never as perfect as the original tail. Regenerated tails do not have bony vertebrae but rather a cartilaginous rod. A few lizards and all snakes do not have such fracture planes and cannot regenerate a tail if it is broken off.

Lizard toes have evolved along many different pathways. Adhesive toe lamellae evolved independently in anoles, geckos, and skinks; improved climbing ability; and led to an arboreal lifestyle. Gecko foot hairs, apparently overdesigned by orders of magnitude, exploit intermolecular Van der Waals forces (the subatomic analogue of gravitational forces) to provide powerful purchase even on very smooth surfaces. (Some geckos can run up a pane of glass.) Similarly, independent acquisition of fringed toe scales in several families (e.g., Iguanidae, Teiidae, Scincidae, and Gerhosauridae) increased traction on soft sands and enhanced climbing ability, even enabling some lizards to run across water.

The highly successful Chamaeleonidae (about 130 species) set their own direction in lizard evolution, taking lingual feeding and sit-and-wait ambush foraging to their logical extremes. Ballistic tongues allow chameleons to capture prey more than a body length away without moving, and having their toes bound together (zygodactly) permits them to hang on to and balance on thin branches to exploit arboreal habitats. Prehensile tails facilitate climbing and are used as a fifth leg. Independently moving, turret-like eyes allow chameleons to look all around without even moving their heads. By capturing their prey without moving, staying completely concealed in vegetation, chameleons have eliminated the riskiest aspect of sit-and-wait ambush foraging—pursuit movements.

Jaw prehension of prey freed the tongue from its role in prey capture, permitting it to evolve to become a specialized chemical sampler, carrying non-airborne chemical particles into the mouth to be received and deciphered by the vomeronasal system. The vomeronasal system was present in squamate ancestors, but it remained relatively rudimentary in most iguanians. In scleroglossans, the foretongue became specialized for protrusion and for picking up and transporting chemical signals, and the skull became less robust and more flexible than in iguanians. The temporal region is less broad, owing to reduction or loss of the upper temporal bar. Additional points of potential flexibility arose in scleroglossan skulls, a condition known as "cranial kinesis," or mesokinesis. This allows the muzzle and upper jaw to flex upward and downward, making the jaws more efficient in capture and